1. Field of the Invention
The invention relates to an effervescent system for effervescent tablets and effervescent granules according to the precharacterizing clause of claim 1, and a process for the preparation of the effervescent system according to the precharacterizing clause of claim 5. Such an effervescent system has been disclosed in U.S. Pat. No. 3,105,792, the content of which is hereby considered to have been disclosed for the purposes of the present description, and according to which alkali metal bicarbonate is heated for a short time to relatively high temperatures or for a longer time to lower temperatures, for example spread over large cups over which air flows.
2. Description of Related Art
It is known that, on the one hand, the stability of sodium bicarbonate to organic acids, for example citric acid or tartaric acid, is reduced by virtue of the fact that, without special measures, reactions occur between sodium bicarbonate and the acid owing to residual moisture which is always present. On the other hand, the sodium bicarbonate tends to undergo thermal decomposition by itself, even at only slightly elevated temperatures, according to the formula EQU 2NaHCO.sub.3 =Na.sub.2 CO.sub.3 +CO.sub.2 +H.sub.2 O.
The reaction with the acid can be prevented--but only partially--by passivating it, i.e. causing it to undergo a partial reaction at the surface with a carbonate or bicarbonate. A further improvement in the stability results if the sodium bicarbonate particles themselves are partially converted at the surface to sodium carbonate, as described in U.S. Pat. No. 3,105,792. The resulting water is at least partially incorporated as water of crystallization in the sodium carbonate, which however is in turn reactive by its very nature.
However, a more exact investigation of the process described in U.S. Pat. No. 3,105,792 showed that it was possible to achieve an improvement compared with commercial alkali metal bicarbonate particles with regard to the stability of the effervescent system and any active ingredients, for example acetylsalicylic acid, using alkali metal bicarbonate particles which are coated with alkali metal carbonate. However, the degree of improvement was still unsatisfactory, particularly in the case of relatively long storage times and/or relatively high temperatures, as may readily occur in hot countries.